Field of the Invention
The present disclosure relates to a network node. In particular, but not exclusively, the present disclosure relates to measures for use in operating a network node in a network.
Description of the Related Technology
Session Initiation Protocol (SIP) devices generally fall into one of three categories as follows:
User agents (UAs) are terminal (or ‘endpoint’ or ‘user’) devices such as mobile (or ‘cellular’) telephones, tablets, laptops, desktop personal computer (PC), etc;
Proxies are network nodes which are typically relatively ‘basic’ (in terms of supported functionality) but high-performance servers; and
Back-to-back User Agents (B2BUAs) are network nodes which are typically relatively ‘advanced’ (in terms of supported functionality) but lower-performance servers.
The difference between a proxy and a B2BUA is that SIP dialogs (communication sessions such as telephone calls) pass through proxies untouched while a B2BUA establishes two (or more) SIP dialogs and bridges between them. Note that SIP is a signaling protocol and reference here is to signaling flows (c.f. the media data for communication sessions might still flow end-to-end even with a B2BUA).
Due to the fact that B2BUAs bridge between two (or more) separate dialogs, they have much more control. In particular, they can initiate new SIP requests. Proxies cannot initiate new SIP requests.
Since a B2BUA has two (or more) separate dialogs, it maintains state for both these dialogs and map between them. A proxy only has a single dialog and so need not maintain state for the single dialog.
Call jump (or ‘call transfer’ or ‘access transfer’) services enables users to ‘jump’ their calls from one device associated with the user to another device associated with the user. For example, the user may receive a call on their desktop PC and then jump it to their mobile telephone because they are about to leave the office. As another example, a user may might start a call over Voice over Long-Term Evolution (VoLTE) (4G) and then access transfer it to 2G/3G upon loss of the 4G signal.
Some SIP flows used in relation to call jump work peer-to-peer between the UAs themselves. Whilst these flows are in general compliant with the Internet Protocol Multimedia Core Network Subsystem (IMS) standards and end-user devices tend to support them, some Interconnection Border Control Function (IBCF) and Media Gateway Control Function (MGCF) network nodes (or ‘network entities’) do not. In the case of ‘access transfer’, the VoLTE specifications mandate that this is implemented in the core of the network.
It would therefore be desirable to provide improved measures for use in operating network nodes, including for example in relation to call jump services.